Blown-film apparatus is used to manufacture plastic bags and the like. A molten tube of plastic is extruded from an annular die and then stretched and expanded to a larger diameter and a reduced thickness by the action of overhead nip rolls and internal air pressure. Where the film is to be wound in a roll, the annular die or the overhead nip rolls are slowly rotated to distribute film thickness irregularities caused by die imperfections. To control the circumference of the finished tube, it is generally necessary to adjust the volume of air captured inside the tube between the annular die and the overhead nip rolls. It has been conventional to adjust the entrapped volume of air by operating valves in the small inflation line connected to the tube interior. These valves were opened or closed in what is known as "bang-bang" control in response to measurements of tube size.
Some of these inflation systems measured tube size using optical beams tangent to the tube and incident upon photocells. For example, Uemura et al. U.S. Pat. No. 3,932,080 shows two photocell limit beams tangent to the oversize and undersize limits of the tube. One photocell opens the on-off valve in the air supply line when its beam is able to pass by the tube and be detected, corresponding to a reduction in tube size. The other cell opens the on-off valve in the vacuum line when its beam is obscured by the tube, corresponding to an increase in tube size. Another reference showing a "bang-bang" photocell system is Suh et al. U.S. Pat. No. 3,159,698. And Tsuboshima et al. (U.S. Pat. No. 3,499,064) and Matsuo et al. (U.S. Pat. No. 3,400,184) each shows a single photocell and beam for controlling the inflation pressure inside the tube.
In certain instances the geometry of the extruding bubble may vary due to shift in the frost line. The invention can be utilized in a number of ways in this event, for instance an elevator may be employed to raise or lower the sensing device to ensure that it is acting upon the tube in a region where meaningful information can be derived.
Mechanical feelers have also been used to control the entrapped volume with "bang-bang" control. For example, Hearns et al. U.S. Pat. No. 3,700,370 uses the voltage difference between the outputs of potentiometers at the feelers to drive a silicon controlled rectifier. A positive difference voltage, corresponding to a reduction in film size, causes the rectifier to energize an on-off solenoid valve in the air line. Sway of the tube, without any change in tube size, does not generate a difference voltage, and thus does not activate the solenoid valve. But using such mechanical feelers leaves deformations in the film.
To increase production speeds of blown film lines, a continuous stream of cooling air has been admitted through passages in the die, directed against the inner wall at the tube and removed through passages in the die. In such dies, in order to regulate the flow rate in a dynamic manner a mechanical feeler that follows the wall of the tube has been used. Schott U.S. Pat. No. 3,980,418, for instance, shows a single feeler mechanically connected to a pneumatic regulator valve which, through a pneumatic cylinder, proportionally controls a flapper valve in the internal air supply line. As with all mechanical feelers, however, deformations are left in the film and a degree of inaccuracy must exist due to the need of the film to apply pressure to the feeler in order to produce a response. Furthermore, with such feelers, tube size measurements may be made beyond the molten region of the tube to avoid serious deformations in the tube wall as the result of cooling by the feelers. Making the measurement away from the molten region introduces delay into the control system, and reduces accuracy.